Automatic gain control circuits



March 29, 1938. K. HAGENHAUS 2,112,279

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LAEcaMm/vENr 0F RECI/F/ED R. ES/GIVALS lNVENTOR KURT HAGENHAUS ATTORNEY Patented Mar. 29, 1938 UNITED- STATES PATENT OFFICE AUTOMATIC GAIN CONTROL CIRCUITS Application September 10, 1936, Serial No. 100,069 In Germany September 10, 1935 8 Claims.

In transmitters it is known that the differences between loud and soft passages of a performance can be decreased in order that disturbing noises will not be too clearly noticed, while, on the other hand, the transmitter will not be subjected to excessive control, i. e., it is not necessary to control it in regard to the loudest sounds. In order that in the receiver the intensity range will again be obtained in its original form, it has been proposed to expand at the receiver end the intensity range by means of a control circuit. Attention has already been drawn to the fact that the control curve for the intensity range at the receiver must exactly correspond to that at the transmitter where importance is to be attached to a true intensity range. It was hitherto overlooked that even in the case of corresponding control curves, the intensity range control in case of high-frequency receivers is not without faults.

In the drawings:

Fig. 1 shows a circuit arrangement employing one form of the invention, and

Figs. 2 to 6 inclusive show respectively different modifications of the invention, and wherein in each modification the gain expansion bias is applied to a radio frequency amplifier.

According to one aspect of the invention, it is proposed to employ, at the same time, gain control preceding and following the control detector, and an intensity range control. In this way it is accomplished that for transmitters differing from each other, as regards intensity or due to fading phenomena, no influence will be exerted upon the intensity range control. This is due to the fact that with gain control acting only upon preceding tubes, the control potential, and hence also the low-frequency potential which at a definite degree of modulation is obtained in the same manner as the control potential from the high-frequency, does not remain constant, since otherwise no controlling action occurs. However, if also a subsequent stage is controlled in a known manner, the low-frequency amplitude depends only upon the degree of modulation of the respective transmitter.

Examples of embodiments of this aspect of the invention are schematically shown in Figs. 1 and 2. In Fig. 1 there is placed in the output of the high-frequency stage HF1, the detector Gs which furnishes the gain control potential (after filtering out the low-frequency) for the preceding high-frequency stage (HF1) and for the subsequent high-frequency stage (HFz). Conse quently, the receiving detector GE always utilizes the same high-frequency amplitudes so that also the low-frequency amplitudes are always the same at the same degree of modulation. As a result thereof, the intensity range control oper-i ates by means of the detector GD which detects the low-frequency energy, entirely independent of the input potentials of the receiver.

In Fig. 2, the intensity range control operates on the high-frequency stages. It would also be possible to combine the arrangements of Figs. 1 and 2. Also other modes of structures are possible, as the high-freequency stage I-IFz could for instance be omitted, and a part of the gain control and also the intensity range control could be carried out in the low-frequency part. In this case, the gain control potential may also be derived from the receiving detector. On the other hand, the detector for the intensity range control may be connected to the output of the detector for the gain control.

In receivers which provide an intensity range 20 control in the audio-frequency network, for instance in order to expand the intensity range of the reproduction while it is compressed at the transmitter, there exists the danger of distortion since the low-frequency amplitudes are very 25 large, and since for controlling the amplification it is necessary to operate on more or less bent parts of the characteristics. This disadvantage is overcome in accordance with another aspect of the invention in that the intensity range con- 30 trol is carried out in the high-frequency stages in which there is no gain control.

Two examples of this phase of the invention are schematically shown in Figs. 3 and 4. In Fig. 3, the control detector Gs for the gain control is connected to the output of the high-frequency stage HF1. Following the filtering out of the low frequency potentials, the control potential acts upon the high-frequency stage I-IF1. But these low-frequency potentials are applied to the detector GD which serves for the control of the intensity range. This control is carried out at the high-frequency stage HFz, and provides expansion of gain thereof.

In the example according to Fig. 4, the lowfrequency produced in the receiving detector GE is applied to the detector Go for the control of the intensity range. The output of this detector is connected to the high-frequency stage HFz. The rectifier Gs provides the usual AVC action.

According to another aspect of the invention, at one or several stages of a high-frequency receiver a gain control, or control of the intensity range is carried out. This phase of the invention has advantages in receivers in which a limited power reserve is available. The gain control and also the intensity range control each require a reserve of power in order that they be fully efiective. Hence, both together can only be used in larger receivers. The construction now to be described affords the possibility of still utilizing the advantages of both controls. In general, the intensity range control will only be connected at local reception, or to receive signals from powerful transmitters not subject to fading phenomena, while, on the other hand, for distance reception more importance is to be attached to compensating the fading phenomena, since in this case the intensity range control would act in a false manner, in view of the changing input potentials.

Examples embodying this aspect of the invention are schematically shown in Figs. 5 and 6. In Fig. 5, following the high-frequency stage HF, a detector Gs is connected for the gain control to correct for fading, and which controls the high-frequency stage. Instead thereof, the control could also be carried out at the receiving detector GE. The low, or audio, frequency energy appearing in the output of the receiving detector will be detected in the detector GD. This control potential will only then be applied to the highfrequency stage HF, when the switch S is in the downward position. The detected audio component is used for expansion of the R. F. amplifier gain.

Figure 6 differs from Figure 5 in that the audiofrequency potential for the intensity range control is derived from the detector Gs, and detected in the detector GD. In accordance with the position of the switch S, a gain control potential, or a potential of the intensity range control, is applied to the highfrequency stage HFz. In this example a gain control is always carried out at the high-frequency stage HF1, so that at the right hand position of switch S, a complete gain control will be effected.

Also, other modes of construction are possible in which, for instance, at an audio-frequency stage, an intensity range control, or an additional gain control, are carried out at will. The arrangement could, also, be such that the switching is carried out automatically in accordance with the input amplitude.

What is claimed is:

1. In a radio receiver of the type including at least two radio frequency amplifiers in cascade, an audio demodulator, an audio amplifier and a reproducer, an automatic volume control circuit responsive to the signal output of the first radio amplifier for controlling the gain of the latter, and an automatic gain expansion circuit, responsive to the audio component of detected signals, for controlling the gain of the second radio amplifier in a sense opposite to the gain control of the first radio amplifier.

2. In a radio receiver of the type including at least two radio frequency amplifiers in cascade, an audio demodulator, an audio amplifier and a reproducer, an automatic volume control circuit responsive to the signal output of the first radio amplifier for controlling the gain of the latter, an automatic gain expansion circuit, responsive to the audio component of detected signals, for controlling the gain of the second radio amplifier in a sense opposite to the gain control of the first radio amplifier, said volume control circuit including a signal rectifier, and said expansion circuit including a rectifier connected to the output of said signal rectifier.

3. In a radio receiver of the type including at least two radio frequency amplifiers in cascade, an audio demodulator, an audio amplifier and a reproducer, an automatic volume control circuit responsive to the signal output of the first radio amplifier for controlling the gain of the latter, and an automatic gain expansion circuit, responsive to the audio component of detected signals, for controlling the gain of the second radio amplifier in a sense opposite to the gain control of the first radio amplifier, said volume control circuit including a rectifier connected to the first radio amplifier output circuit, and said expansion circuit including a device for rectifying the audio component of the rectified output of said rectifier.

4. In a radio receiver of the type including at least two radio frequency amplifiers in cascade, an audio demodulator, an audio amplifier and a reproducer, an automatic volume control circuit responsive to the signal output of the first radio amplifier for controlling the gain of the latter, an automatic gain expansion circuit, responsive to the audio component of detected signals, for controlling the gain of the second radio amplifier in a sense opposite to the gain control of the first radio amplifier, said expansion circuit including a rectifier for rectifying the said audio component, and means for applying the rectified audio component to the second radio amplifier.

5. In a radio receiver of the type including a radio frequency amplifier network, a demodulator, an audio frequency amplifier network and a reproducer, means responsive to variations in modulated signal carrier amplitude for automatically controlling the carrier transmission efficiency through the radio amplifier network in a sense to maintain the carrier amplitude at the demodulator input substantially uniform regardless of fading at the radio amplifier network input, and additional means, responsive to an increase in the audio component amplitude of the modulated signal carrier, for increasing the said transmission efliciency.

6. In a radio receiver of the type including a radio frequency amplifier network, a demodulator, an audio frequency amplifier network and a reproducer, means responsive to variations in modulated signal carrier amplitude for automatically controlling the carrier transmission efiiciency through the radio amplifier network in a sense to maintain the carrier amplitude at the demodulator input substantially uniform regardless of fading at the radio amplifier network input, additional means, responsive to an increase in the audio component amplitude of the modulated signal carrier, for increasing the said transmission efiiciency, said first means including a carrier rectifier, and said additional means including a rectifier for said audio component.

'7. In a radio receiver of the type including a radio frequency amplifier network, a demodulator, an audio frequency amplifier network and a reproducer, means responsive to variations in modulated signal carrier amplitude for automatically controlling the carrier transmission efficiency through the radio amplifier network in a sense to maintain the carrier amplitude at the demodulator input substantially uniform regardless of fading at the radio amplifier network input, additional means, responsive to an increase in the audio component amplitude of the modulated signal carrier, for increasing the said transmission efiiciency, said first means including a rectifier upon whose input is impressed modulated carrier energy, and said additional means deriving its audio component input from said rectifier output.

8. In a radio receiver of the type including a radio frequency amplifier network, a demodulator, an audio frequency amplifier network and a reproducer, means responsive to variations in modulated signal carrier amplitude for automatically controlling the carrier transmission efiiciency through the radio amplifier network in a sense to maintain the carrier amplitude at the demodulator input substantially uniform regardless of fading at the radio amplifier network input, additional means, responsive to an increase in the audio component amplitude of the modulated signal carrier, for increasing the said transmission eificiency, said radio amplifier network comprising at least two amplifier stages in cascade, said first means controlling the gain of the first stage, and said additional means controlling the gain of the second stage.

KURT HAGENHAUS. 

